The present invention relates to multilayer reflective films, particularly to high reflectance and low stress multilayer films, and more particularly to Mo.sub.2 C/Be multilayers having low stress and high reflectance, and which may include a carbon capping layer.
Multilayer reflective mirrors for soft x-ray to ultraviolet wavelength have been under development for several decades. These early multilayer efforts involved layers of a transition metal and layers of a compound of carbon and silicon, boron, beryllium, or aluminum, and resulted in a reflectance of about 55%. Such a multilayer system is exemplified by U.S. Pat. No. 5,310,603, issued May 10, 1994 to Y. Fukuda et al.
In recent years, extensive efforts have been directed to the development of extreme ultraviolet (EUV) lithography tools or systems which operate in the 11-13 nm wavelength regime. Such systems require several precisely figured, low roughness optics coated with highly reflective multilayers. To obtain sufficient throughput and image quality, these multilayer coatings should simultaneously have high reflectance, preferably near 70%, and low stress (less than 100 MPa), and have high thermal and radiation stability.
Until recently, the only multilayer material pair that would reflect near 11 nm and appear to have the potential to satisfy the requirement for high reflectance, low stress, and long term stability, was Mo/Be. However, this material combination had problems relative to long term stability and stress. The Mo/Be multilayer development efforts are exemplified by: K. M. Skulina et al., Molybdenum/beryllium multilayer mirrors for normal incidence in the extreme ultraviolet, Applied Optics 34, 3727 (1995); D. G. Stearns et al., "Beryllium-based multilayer structures, Mat. Res. Soc. Symp. Proc. 382, 329 (1995) Materials Research Society; T. W. Barbee Jr. et al., Interfacial Effects in Multilayers, SPIE 1997; and C. Montcalm et al., Multilayer reflective coatings for extreme-ultraviolet lithography, Emerging Lithography Technologies II, Y. Vladimirsky, ed., Proc. Soc. Opt. Instrum. Eng. 3331, paper 331-05 (1998).
The present invention involves a new material pair (Mo.sub.2 C and Be) to make EUV multilayer coatings. The Mo.sub.2 C layers are nano-crystalline (amorphous). The formation of a beryllium carbide at the Mo.sub.2 C/Be interfaces both stabilizes and has a smoothing effect on the interface. Tests have indicated that the interfaces appear to be smoother than the interfaces in Mo/Be multilayers, and such interfaces are expected to be more stable, based on the Mo.sub.2 C/Si study by T. W. Barbee Jr. et al., referenced above. To reduce the reduction in reflectance due to oxidation of the outer layer, a carbon capping layer is deposited, which has been shown to increase the reflectivity by about 7%, as compared to the multilayer where the outer layer has been oxidized. The reflectance of the carbon capped Mo.sub.2 C/Be multilayers is above 65% at 11.25 nm with a tensile stress of +88 MPa, compared to a stress of +330 MPa of Mo/Be multilayers of the same thickness. The Mo.sub.2 C/Be multilayer system of the present invention can be deposited using first and last Be layers, or a first layer of Be and last Mo.sub.2 C layer and capping the Mo.sub.2 C layer with a thin carbon layer.